Presently, when air bag covers are provided in automobiles on the driver side of the vehicle, the air bag is stored in the steering column behind an air bag cover. During automatic inflation of the air bag, the air bag cover moves away from the steering column to permit its safety function between the steering column and the operator of the vehicle. Air bag covers may also be provided on the passenger side of the vehicle as well.
Recent practice in the automotive industry is utilization of all plastic fabricated air bag covers. Conventional air bag covers used in conjunction with occupant restraint systems often include noticeable or visually perceptible tear seams or scores disposed on the exterior surface of the air bag cover. The tear seams or scores represent selected weakened surfaces where the inflating air bag initially separates or breaks through the air bag cover and moves away from the steering wheel to perform its safety feature.
Typically, a tear seam is formed into an air bag cover by injecting plastic into an injection mold in which the mold cavity includes a projection defining the tear seam. An example is illustrated in U.S. Pat. No. 5,335,935 issued to Proos et al. However, with this method it is difficult to inject the plastic over the projection, resulting in swirl lines caused by turbulence. Also, it is very difficult to have molten plastic flow over the projection, therefore, the thickness of the tear seam is limited, typically 0.4 millimeters or more.
U.S. Pat. No. 5,320,380, issued to Hamada et al., discloses an air bag cover having a hidden tear seam of less than 0.4 millimeters. However, this air bag cover is manufactured utilizing RIM (Reaction Injection Molding) technology. With RIM, a mold cavity supporting a mesh screen is injected with two liquid materials that chemically react to form a thermoset product. However, a product manufactured utilizing RIM molding is relatively more expensive, less durable and heavier than a product manufactured utilizing thermoplastic injection molding.